Who was a vice president under John Quincy Adams?

John Calhoun served as vice president under John Quincy Adams. This is significant because Calhoun was a prominent political figure of his time, known for his strong advocacy of states' rights and his role in the Nullification Crisis. The position of vice president during Adams' administration (1825-1829) was critical as it represented a complex political environment, where Calhoun initially aligned with Adams before conflicts arose concerning their differing political ideologies. This relationship set the stage for Calhoun's later political maneuvers and his eventual break with the Adams administration as he shifted his allegiances towards Andrew Jackson, representing the changing dynamics and factions within early American politics.

The other options do not align with the correct historical context of who served as vice president during Adams’ presidency. Henry Clay was a prominent political figure but did not serve as vice president. Andrew Johnson became vice president much later, under Abraham Lincoln, and James Buchanan was a later president himself, who served after Adams. Thus, the choice of John Calhoun accurately reflects the historical record.